1. Field of the Invention
This invention relates to a method of and an apparatus for radio monitoring. More specifically, the present invention relates to a technique for specifying the position of a radio wave source such as an unlicensed radio station in a particular region, monitoring the utilization environment of the radio wave resources and identifying and visualizing a radio wave source when such a radio wave source is found out.
2. Description of the Related Art
Conventionally, in order to search out a radio wave source such as an unlicensed radio station and specify its position, a plurality of sensor stations (radio monitoring facilities) are arranged at different points, and the sensor stations observe incoming directions of the radio wave from the radio wave source by using Yagi-Uda antennas or goniometers. The position of the radio wave source is estimated by plotting the incoming directions at the sensor stations on a map to detect the position of a cross point (see, for example, Herndon H. Jenkins, "Small-Aperture Radio Direction-Finding", Artech House, 1991). In order to actually prosecute an unlicensed radio station, after the position of the unlicensed ratio station is specified in such a manner as described above, it must be determined which apparatus or which object at the position is the actual radiation source of the radio wave as an unlicensed radio station. In this sense, to determine the position on a map or the geographical position of the radio wave source is called specification of the position of the radio wave source, and to identify which object at the position is an actual radio wave source is called identification of the radio wave source.
The conventional method described above assumes that a radio wave from an objective radio wave source propagates straightforwardly along a single path to a sensor station. However, a radio wave from the source is sometimes diffracted or reflected by geographical features or objects on the ground such as buildings and then propagates without advancing straightforwardly. Further, a radio wave from a radio wave source sometimes comes to a sensor station following a plurality of different propagation paths (multipath) by an influence of the geography or objects on the ground. When the position of a radio wave source which is an object of monitoring is estimated based on the conventional method, errors occur in the detection of the incoming direction of the radio wave, and this gives rise to a problem that the position of the radio wave source cannot be estimated with a high degree of accuracy. For example, if the incoming directions of a radio wave are measured at three or more sensor stations, then the plotted incoming directions may not cross at a single point. Where the accuracy in measurement of the position of the radio wave source is not very high, there is another problem that also it is difficult to identify the radio wave source.
With the conventional method, since a horizontal search of an incoming direction is performed without using information with regards to the vertical direction, there is a problem also in that it is difficult to specify such a radio wave source that is located in a room of a high- or medium-rise building. Further, with the conventional method, a result of measurement obtained is a field strength at a location of a sensor at which the radio wave is received, and if a different radio station which operates with the same frequency is present, then the sensor may receive the objective radio wave combined with the radio wave from the different radio station. Accordingly, the conventional method also has a problem in that the result of measurement cannot be treated easily as objective numerical data for the transmission power of the radio wave source.
The aim of radio monitoring is not only finding out of an unlicensed radio station but also to provide better utilization environment of radio wave resources by investigating whether or not radio wave interference occurs or a cause of such radio wave interference. In order to investigate a cause of radio wave interference, it is effective to find a propagation path from a radio wave source and a field strength distribution. However, the conventional method described above has a problem also in that it cannot determine how a radio wave from a radio wave source propagates to a sensor station nor determine a field strength distribution of the radio wave from the radio wave source irrespective of whether or not the radio wave source is a radio station which operates legally or is an unlicensed radio station.